Power Line Communication
That may not mean a lot to you until you know what it is. Power Line Communication seemingly works by magic.
Problem: You have a wireless router which works with your wireless laptop and your Mum can sit on the sofa browsing on her iPad and your sister can download games on her phone. But your XBOX 360 and your OnLive console which you want to use in your room upstairs – away from the router – and only have ethernet Internet (technically LAN) connections.
Solution: Buy two Power Line Communication boxes which can sent wired ethernet data through your plug sockets – plug one in by your router, one in by your wired device, and connect them both up like you normally would! See? Magic!
Whenever you tell anyone about this, it usually shocks them – I mean theoretically it should work, but in practice, you just can’t see it working very well. In actual fact you don’t get bad throughput – roughly 80% of what you would normally get over a CAT5 cable.
It actually works by using a higher frequency over the same cables the power uses, and the transformers in the plugs themselves take care of the all the differences. You can read more on the wiki page: http://en.wikipedia.org/wiki/Power_line_communication
Even thought they are expensive, they have come down in price a lot from when I last looked. These, for example look pretty good for their money, and they look quite tidy too: http://www.amazon.co.uk/TP-Link-TL-PA211KIT-200Mbps-Powerline-Ethernet/dp/B004INVKP4/ref=sr_1_5?ie=UTF8&qid=1319050256&sr=8-5
Update: Will Derrick pointed out to me that you really should change the default password on the set of these if you happen to get any, otherwise anyone who can actually see them might be able to compromise your security!
just in the shepherdnick of time 
I can confirm this works, our router is downstairs below my room, so I get wireless from the main router to my desktop.
My brothers room is on the other side of the house and couldn’t get a clear signal, so he got power line converters put in, and also a router of his own for the incoming signal, so he could have multiple wireless connections off it in his room. His connection now seems fine.
Mine however is not that good, because the router originally was upstairs and in Ethernet cables distance to my PC, and because Rob couldn’t get a good connection via wireless, they decided they would sort it fine for him and take away my Ethernet connection, and reduce me to crap wireless instead. Didn’t even give me chance to say “why don’t you just get a longer cable and run it to Rob’s room?”